Why not just shut off the breaker though? What’s the harm in that?

Live feedback without additional expensive and buggy test tools. May be marginal, but if you are trying to guarantee a certain income/payrate, it’s easy to take shortcuts in a system you’re comfortable in.

Other devices on the same circuit that I didn't want to disturb.

For effect. You want the client to feel that they got their money's worth and it wasn't something they could have done themselves.